VIRULENCE OF THE BACTERIOPHAGE 141 



virulence of a pathogenic bacterium? Simply the power to develop 

 within and at the expense of the host, and we consider the degree of 

 virulence to be the higher as this development is the more rapid. Logi- 

 cally, then, if these definitions are correct that which we have termed 

 "activity" in a bacteriophage corpuscle represents a "virulence" in 

 the strictest sense of the word (d'Herelle^^^- ^^^), It is evident, and not 

 without interest, that the term "virulence" as apphed to the bacterio- 

 phage is employed in this same sense by those authors (Otto, for ex- 

 ample) ^^* who still consider the bacteriophage to be a ferment. 



2. EVALUATION OF THE VIRULENCE OF A BACTERIOPHAGE 



The virulence of the bacteriophage being variable from one race to 

 another, it is desirable to be able to express numerically the virulence 

 of a given race. Let us consider the methods which have been proposed, 

 and evaluate them with regard to their precision and their utility. 



From the very first of my studies I have advocated and employed 

 solely the method involving an enumeration of the corpuscles. To this 

 end I have carefully spread upon an agar slant in a 22 mm. tube, 0.02 

 cc. of a suspension of the susceptible bacterium containing 250 million 

 bacteria per cubic centimeter inoculated with a dilution of the bac- 

 teriophage of such a titre that, after incubation, the plaques are iso- 

 lated.^-^ In some instances, as an alternative procedure, I have used 

 Petri dishes, and in this case I have spread 0.05 cc, or 0.02 cc. of the 

 suspension, according to the size of the plate.^^^ This method is the 

 only one which should be employed for the study of the phenomenon of 

 bacteriophagy if false interpretations, associated with poor methods of 

 evaluating the bacteriophage, are to be avoided. 



When it is not essential to obtain results of the greatest accuracy, for 

 example, when it is desired simply to observe the variations in virulence 

 shown by the bacteriophage as isolated from the body at different stages 

 of a disease and during convalescence, a more simple and rapid method 

 may be employed, based upon the development of bacteriophagy in a 

 liquid medium and on the general appearance of agar sub-cultures.^-^ 



Since it will be necessary, in many cases throughout this discussion to 

 indicate the relative degree of virulence possessed by a given race of the 

 bacteriophage, it may be well to indicate here a method to express this 

 virulence. This system is somewhat arbitrary, but it meets aU practical 

 needs, and will facilitate expression. 



= no virulence toward a given bacterium. Normal cul- 

 tures of the bacterium develop in bouillon or on agar. 



